From the following documents, namely
German periodical Acta Neurochir. 1993 (Suppl.) 59, pp 50-57; PA1 German periodical Medizintechnilc 110 vol 2. 1990, pp 44-53 (special issue) PA1 Acta Neurochirurgica 1992 (Suppl.) 59, pp 50-57 PA1 Periodical Biotelemetry Patient Monitoring 1979, pp 16-31 PA1 German Periodical Proceedings Dtsch. Ges. Biomed. Techn. (1973), pp 29, 30 PA1 "Sauerstoffsensoren fur in-vivo Messungen" ["Oxygen sensors for in-vivo measurements"], W. Mundt, PA1 Laborpraxis, July-August 1984, pp 736-739 PA1 "Ein neues Sauerstoff-Me.beta.system" [A new oxygen measuring system] Klaus Rommel PA1 Medical and Biological Engineering and Computing, 30, 1992, pp 121-122 PA1 U.S. Pat. No. 4,950,378
page 51 and under the title "Brain Tissue pO.sub.2 Measuremerts", further
and
it is known to perform measurements directly in the brain tissue using a thin flexible catheter probe of the Clark type.
From the following documents, namely
and
very thin and highly flexible tubular catheters are known which form a Clarik type pO.sub.2 probe suitable for this purpose.
Using appropriate emplacement techniques such as are described in the post-published patent application DE 195 02 183.5, such probes can be freely placed in the brain. Because of their small diameters and high flexibilities, such probes are suitable for long-term measurements in the brain tissue and in other susceptible body tissues. They are able to follow tissue movement (pulse, breathing) without hampering such movements and apply only slight pressure effects on the surrounding tissue, and as a result the oxygen supply is not adversely affected in the measurement process.
On the other hand, such extremely thin probes have drawbacks because their slight dimensions affect the implementability of long-term measurements.
Typically, a probe of the above type has an inside diameter of a few tenths of a mm and a length of several tens of cm. Consequently, the inner space to be filled with electrolyte is of the order of a few mm.sup.3, of which however only a small portion is available in the zone of the tiny cathode surface Such a small electrolyte supply is unusually low for PO.sub.2 probes of the Clark type and this constitutes a drawback for long-term measurements.
Clark type pO.sub.2 probes subject an electrolyte space to the ambient oxygen by means of an oxygen diffusion membrane. The oxygen diffuses through the membrane against the diffusion impedance. The quantity of oxygen per unit time entering the electrolyte depends on the ambient partial oxygen pressure. The entering oxygen is reduced by two electrodes, namely a cathode made of a suitable noble metal and an anode typically of silver, optionally with a silver oxide surface. The resulting current (measuring apparatus current) unambiguously depends on the in-diffusing oxygen, that is the ambient partial oxygen pressure. An appropriate polarographic potential must be applied to the electrodes.
Salt solutions, usually sodium chloride, are used as the electrolyte. Hydroxide is generated in the reduction of oxygen. Anodic silver goes into solution as silver salt. Silver is indeed amply present at the anode. On the other hand, the tiny supply of electrolyte of the probes being discussed is consumed comparatively quickly. The electrochemical equilibria change, and the relationship between the measuring current and the incoming flow of oxygen changes. When this occurs, the measurement must be stopped and the probe must be replaced. Electrolyte replenishment at the site, that is in the tissue, is impossible.
The problem of electrolyte consumption is described for probes of this kind in the German periodical
and a 3-electrode configuration is proposed to solve this problem, which however is unsuited for probes used in the brain because of the required minute diameter and high flexibility, and moreover such a system can only sense the electrolyte consumption but not eliminate it.
A medical pO.sub.2 probe is known from the British periodical
which while basically operating on the Clark principle on the other hand does not comprise the membrane rigorously permeable only to gases, but a polyurethane membrane. This material is water-permeable, as indeed required in this design, in order to soak the electrolyte salt which is present in dry form under the membrane. Because the membrane, however, is also ion-permeable, the probe responds to the ambient pH and measuring is possible only at constant ambient pH. Therefore, the measurements are to be carried out in a phosphate buffered solution. This probe is poorly suited for medical purposes because it responds to ambient, fluctuating pH and moreover may release salts from its electrolyte supply to the outside and thereby may damage the tissue.
A probe used in sugar determination in a biological environment is known from
calling for an enzyme converting sugar into H.sub.2 O.sub.2 by means of an intrinsic probe operating on the Clark principle. Poisoning of the cathode surface takes place in this probe and can be eliminated by temporary operation at reversed polarity. This probe is free of electrolyte-supply problems.